DISPERSAL 149 



it to migrate ; although, without accompanying increase in 

 numbers, the species would not spread far. Ritchie says : 

 " Dr. Harvie-Brown was of opinion that the capercaillie 

 viewed prospective sites from its old establishments, and this 

 very probable selection by sight, together with the fact that 

 most of the woodland lay along the watercourses, would 

 determine the capercaillie's dispersal along the valleys. 

 Indeed, judging from the dates of the advent or establishment 

 of birds in new areas, the valley systems ranked second only 

 to the presence of fir woods in determining the course of the 

 migrating capercaillies." The squirrel, which was also intro- 

 duced into Scotland, behaved in a similar way, migrating along 

 the valleys and frequenting the fir woods.-^^^ 



4. So far we have used the term " dispersal " in a rather 

 loose way. It is necessary, however, before entering further 

 upon the subject, to analyse the process of dispersal and define 

 rather carefully certain terms which are usually used in a 

 vague sense, or at any rate in different senses by different 

 people. The following system of terms is used here, and 

 whether or not it agrees with the definitions of other writers, 

 it will at least be clear what is meant by them here. By 

 " dispersal " is meant the actual migration or carriage of 

 animals from one place to another : e.g. the floating of young 

 spiders on streamers of gossamer, the flight of a migrating 

 goose, the transport of water-mites on the legs of water- 

 beetles, or the drifting along of jellyfish in the sea. When 

 such an animal reaches its destination it may either die or 

 survive. It usually dies, but if it does not, we say that it has 

 " established itself as an individual." Thus, a large logger- 

 head turtle {Thalassochelys caretta) reached the coast of 

 Scotland (Skye) in December, 1923, quite alive, and full 

 of eggs.^-^ This species is an inhabitant of tropical and 

 subtropical seas. Again, in certain years large numbers of 

 clouded yellow butterflies (Coltas edusa) arrive in England 

 from the Continent. But in such cases the individuals are 

 unable to breed successfully, or else the young are unable 

 to survive. The next stage is, therefore, that the animal 

 must *' establish itself as a species." It does this if it is 



